Means by which walls and the like may be insulated are much discussed in the literature, and various ways of minimizing conductive, convective and radiative heat transfer are described. Fundamentally, all approaches use certain basic understandings of heat transfer. These understandings are applied to select and combine materials in a way which provides the highest possible insulating value under a given set of conditions. There is always a trade-off, however, with respect to the cost of the materials used, and the effort necessary to manufacture such materials into a truly effective insulator from both an insulating and cost of manufacture standpoint.
As the cost of energy increases, even relatively small quantitative improvements in the total performance of a particular insulating scheme can provide significant savings for the energy consumer. Given the same insulating value, the relative value of an improved insulating scheme must take into account the thickness of the material, the cost of the materials used, the cost to assemble those materials in the proper form, the cost of transporting such insulation to the place of installation, and the cost to install. Generally, insulations with good thermal performance per unit thickness are more desirable. Bulky and or heavy insulation schemes are disadvantageous, even if good insulating value is provided. Also disadvantageous are materials which have negative environmental impacts, such as chlorofluorocarbon (CFC) blown foams, and potential health hazards, such as airborne fiberglass.
Other characteristics are desirable for certain applications. Daylighting schemes employ natural light transmitted through a building envelope. For these applications it is useful to have an insulator that is partially transparent to the visible spectrum but at the same time providing good resistance to heat flow.
Retrofit applications typically have limited space available and/or difficult access to these spaces. For such cases it is especially desirable to have an insulator that is high performance per unit thickness to attain overall performance comparable to current accepted levels of insulation for new construction. In addition, the insulator must be easily installed and functional in limited access spaces, for example by inserting collapsed and expanding to fill a mostly closed stud wall cavity.
Insulation performance is often measured by use of "R" values, where R is a thermal resistivity, and higher R-values indicate better insulating performance. R-value/in. is used to compare the thermal performance of different insulating materials. For example, fiberglass has an R-value/in. of about 3.2 hr-ft.sup.2 -F/BTU, while styrene foam has an R-value/in. of about 5 hr-ft.sup.2 -F/BTU. Chlorofluorocarbon (CFC) blown polyurethane foam has an initial R-value/in. of about 7.2 hr-ft.sup.2 -F/BTU, which slowly degrades as CFCs diffuse out of the foam cells.
The primary function of a thermal insulation is to reduce heat transfer. There are three forms of heat transfer; conduction, convection, and radiation. Conduction involves heat flow through the material in the form of direct interaction of atoms and molecules. Convection involves conduction combined with mass transfer of fluids (gases) where heat flow is enhanced by the relative movement of fluids at different temperatures. Radiation involves direct net energy transfer between surfaces (at different temperatures) in the form of long wave infrared electro-magnetic radiation (light) as surfaces emit and absorb this radiation. The amount of absorption and emission of radiation depends on the surface emissivity which is a material property. Low emissivity surfaces emit and absorb much less radiation than high emissivity surfaces.
Insulations attempt to reduce these three components in different ways. Fiberglass insulation utilizes fairly low conductivity fibers in a stack or batt with air amongst the fibers. The fibers are oriented, somewhat randomly, such that they do not line up in the direction across the batt thereby reducing solid conduction. Convective movement of the air amongst the fibers is reduced by the presence of large numbers of fibers. Radiation is somewhat scattered as it passes through the fibers which reduces radiative heat transfer. Closed cell foam structures, however, are comprised of a polymer matrix with many small, mostly closed cavities. Conduction is reduced by using low conductivity materials and comprising most of the volume with air (or remains of the blowing agent). Convection is effectively eliminated by trapping the gases in small closed cells. Radiation is low because the cells are typically very small and there is little temperature difference between cavity walls and hence low driving force for radiative heat transfer.
Conduction can only be eliminated by removing all the mass, as in a vacuum, which is difficult to produce and maintain. Conventional insulations utilize gas for most of the volume because gases are much lower conductivity than solids or liquids. The ideal limiting case for gaseous insulations is when convection and radiation are completely suppressed and the only form of heat transfer is through the gas. Still gas conductivities in terms of R-value/in. for some gases at two different temperatures, are for example:
______________________________________ 273 K. (32 F.) 300 K. (80 F.) ______________________________________ Air 6.0 5.5 Argon 8.8 8.1 Krypton 16.6 15.3 ______________________________________
These values constitute maximum performance potentials for gaseous based insulations. It is known that higher performance insulation is obtainable by using gases of lower conductivity than air. Current insulations are incapable of utilizing harmless inert gases such as argon and krypton. This is a problem for existing insulation schemes because they are limited to relatively low thermal performance per unit thickness. As energy efficiency becomes more important it is desirable to use insulations of better performance per unit thickness, rather than thicker layers of existing insulations.
The invention herein minimizes the three forms of heat transfer by different approaches than existing insulations. Conduction is minimized by not only comprising the panel mostly of gas but the designs also allow for the use of any type of gas desired. Conduction can then be reduced by the use of gases with lower conductivity than air. Convection is suppressed by the use of continuous solid layers in the form of films or sheets in an assembly that creates cavities. The cavities are sized and arranged such that convective heat transfer is effectively eliminated. Radiative heat transfer is reduced by the use of low emissivity surfaces on the layers forming the cavities. The combination of convection and radiation suppression is intended to achieve performance very close to the ideal still gas conductivities.
It is thus an object of the present invention to provide optimal baffle forms that are practical to produce, and perform very close to the ideal still gas by 1) suppressing convection/mass transfer effects by utilizing continuous solid layers in the form of films or sheets in an assembly and 2) suppressing radiative heat transfer by the use of low emissivity surface cavities. Baffle forms have the effect of being a useful insulation medium that can be effectively filled with a low conductivity gas and maintained as such with a gas barrier envelope surrounding the baffle forms thus forming a panel.
It is a further object of the present invention to provide an insulating panel having as much or more insulating value per unit thickness as those schemes now commonly in use, is capable of making excellent use of low conductivity gases, in a form which is simple to manufacture, is extremely lightweight and is capable of being produced in collapsed form to minimize the cost of transportation. The insulation panel must be compatible with current construction methods for new walls and ceilings, in manufactured housing and for site built structures. The insulation panel must also be capable of being used in many retrofit applications where other forms of insulation are either impossible or too low performance.
It is another object of the invention to provide an insulation panel which can enable a significant increase in overall wall thermal performance without an increase in wall thickness or a significant change in construction technique.
It is another object of the invention to provide effective insulation panels and techniques for a broad range of insulation applications within the temperature ranges found in buildings and the appliances associated with them.
It is another object of the invention to provide an effective insulator that can be produced with available materials and processing technologies.
It is another object of the invention to provide an insulation panel that is harmless to handle, has an appealing appearance, and is not otherwise messy or unattractive.
It is another object of the invention to provide a stiff insulating panel which may be retrofit or built into cold storage container walls, such as refrigerators and freezers which is long lived, high performance, which avoids the use of CFCs, and which can be used in composite with non-CFC foams.
It is another object of the invention to provide an insulation technique which can be incorporated into durable goods, such as entire refrigerator/freezer components, made by employing baffle forms of the invention in structural/barrier polymer encasements.
It is another object of the invention to produce an insulating material which can be formed in significant part from recycled, and recyclable materials.
It is a further object of the invention to produce a panel which is at least partially transparent to visible light, but which is nevertheless highly insulating and retains all the insulating characteristics of opaque panels.